The present invention relates to spatial light modulators. The invention relates more particularly to low voltage MEMS-based (micromachined electromechanical system) electrostatic actuators and MEMS-based phase spatial light modulators utilizing the same.
The use of high performance spatial light modulators (SLMs) for military and commercial adaptive optics (AO) applications is increasing rapidly. Examples are free-space communication systems corrected for atmospheric distortion, compensated imaging systems, secure holographic storage systems, optical computers, and various defense applications, among others. Key SLM characteristics are high speed, high phase resolution, large number of pixels, small size/weight, and low cost. SLMs based on micromachined electromechanical systems (MEMS) technology have received much attention for their potential to achieve the desired performance parameters while greatly minimizing size. For example, size minimization is typically achieved by integrating the electronic driver for each pixel with the MEMS structure.
MEMS AO designs have typically been based on conventional parallel plate electrostatic actuators as shown in FIG. 1, and generally indicated at reference character 100. The actuator 100 is shown having two parallel plates 101 and 102 which are spaced apart with an electric voltage 104 applied therebetween. While not shown, the lower plate 101 is typically fixed, while the upper plate 102 is movable in a direction normal to the surface of the plate. Additionally, the conventional parallel plate electrostatic actuator is modeled to account for mechanical energy storage, such as potential energy storage in a flexible plate or a spring connected to a rigid plate (see 105), or the kinetic energy generated by plate movement. As shown in FIG. 1, the movable plate 102 is typically a mirror having a reflective surface 103 to reflect incident light 106, as indicated by arrow 107. As an alternative (not shown) a mirror is often connected to the movable plate by means of a connecting anchor, in order to prevent mirror warpage due to plate movement.
In the conventional parallel plate electrostatic actuator of FIG. 1, applying a voltage 104 operates to attract the two plates together and thereby move the upper plate downward. However, the drive voltage necessary to actuate these parallel plate electrostatic actuators are very high, typically on the order of 50-200 V or higher depending on the required plate motion and the response time. Even though MEMS SLMs do not draw significant current, the high voltage requirement has various drawbacks. For example, the high voltage requirement necessitates the use of large high voltage transistors where only limited space is available. For the scaleable architectures required in many applications involving thousands to millions of individually controlled pixels, electronics must be integrated with the MEMS mirrors. Because pixel sizes are on the order of about 100 microns on a side, there is limited space below the mirrors to incorporate drive electronics. High voltage transistors are much larger than low voltage transistors used in common integrated circuits, making it challenging to integrate the required circuitry. Moreover, the large space requirements of high voltage transistors leave little or no room for other control electronics, such as feedback control circuits. And high voltage power supplies typically dominate the overall system size. The power dissipation from high voltage drivers is also a concern, as temperature gradients may cause mirror warpage. And with high voltage drivers, active cooling may be required which further increases system complexity, size, and cost.
Another significant drawback of conventional parallel plate electrostatic actuators is their highly non-linear response, which must be compensated for through additional electronics. FIG. 2 shows a graph illustrating the strongly non-linear displacement-versus-voltage characteristics typically seen in present SLM devices, (e.g. utilizing a conventional parallel plate electrostatic actuator). Such SLM devices therefore require high precision electronics drivers for adaptive control and error correction, further increasing complexity, size, and cost.
In summary, therefore, there is a need for a MEMS-based SLM, as well as a MEMS-based electrostatic actuator, designed to require lower drive voltages to reduce system cost and size, while further enhancing performance parameters such as response linearity.
One aspect of the present invention includes an electrostatic actuator associated with a pixel in a spatial light modulator having a plurality of pixels. The actuator comprises: a lower electrode; an upper electrode fixed with respect to the lower electrode; a center electrode suspended and actuable between the upper and lower electrodes, and having resiliently-biasing means for restoring the center electrode to a non-actuated first equilibrium position; a mirror operably connected to the center electrode; a first voltage source for providing a first bias voltage across the lower and center electrodes; a second voltage source for providing a second bias voltage across the upper and center electrodes, wherein the first and second bias voltages determine the non-actuated first equilibrium position of the center electrode; and a third voltage source for providing a variable driver voltage across one of the lower/center and upper/center electrode pairs in series with the corresponding first or second bias voltage, for actuating the center electrode to a dynamic second equilibrium position.
Another aspect of the present invention includes a spatial light modulator (SLM) comprising: an array of electrostatic actuators, each representing a pixel of the SLM and comprising: a lower electrode; an upper electrode fixed with respect to the lower electrode; a center electrode suspended and actuable between the upper and lower electrodes, and having resiliently-biasing means for restoring the center electrode to a non-actuated first equilibrium position; a mirror operably connected to the center electrode; a first voltage source for providing a first bias voltage across the lower and center electrodes; a second voltage source for providing a second bias voltage across the upper and center electrodes, wherein the first and second bias voltages determine the non-actuated first equilibrium position of the center electrode; and a third voltage source for providing a variable driver voltage across one of the lower/center and upper/center electrode pairs in series with the corresponding first or second bias voltage, for actuating the center electrode to a dynamic second equilibrium position.
Another aspect of the present invention includes a MEMS electrostatic actuator comprising: a first electrode layer; a second electrode layer fixed with respect to the first electrode layer; a center electrode layer suspended and actuable between the first and second electrode layers and having resiliently-biasing means for restoring the center electrode layer to a non-actuated first equilibrium position; a first voltage source for providing a first bias voltage across the first and center electrode layers; a second voltage source for providing a second bias voltage across the second and center electrode layers, wherein the first and second bias voltages determine the non-actuated first equilibrium position of the center electrode layer; and a third voltage source for providing a variable driver voltage across one of the first/center and second/center electrode layer pairs in series with the corresponding first or second bias voltage, for actuating the center electrode layer to a dynamic second equilibrium position.
Another aspect of the present invention includes a process for controlling a spatial light modulator (SLM) comprising the steps of: providing a SLM having a plurality of electrostatic actuators, each actuator having a lower electrode, an upper electrode fixed with respect to the lower electrode, a center electrode suspended and actuable between the upper and lower electrodes with resiliently-biasing means for restoring the center electrode to a non-actuated first equilibrium position, and a mirror operably connected to the center electrode; for each electrostatic actuator, (a) providing a first bias voltage across the lower and center electrodes, and a second bias voltage across the upper and center electrodes, to establish the corresponding non-actuated first equilibrium position; and (b) applying a variable driver voltage across one of the lower/center and upper/center electrode pairs in series with the corresponding first or second bias voltage to actuate the center electrode to a dynamic second equilibrium position, wherein the variable driver voltage is applied independent of other actuators.
Another aspect of the present invention includes an electrostatic actuation process comprising: providing first and second electrodes fixed relative to each other and a center electrode suspended and actuable between the first and second electrodes, the center electrode having resiliently-biasing means for restoring the center electrode to a non-actuated first equilibrium position; providing a first bias voltage across the first and center electrodes, and a second bias voltage across the second and center electrodes to establish the non-actuated first equilibrium position; and actuating the center electrode to a second dynamic equilibrium position by applying a variable driver voltage across one of the first/center and second/center electrode pairs in series with the corresponding first or second bias voltage.